As of August 2018, the series is composed of nine novels and 53 short pieces (short stories and novellas), all of which fit into a fictional timeline stretching from the Big Bang singularity of the past to the eventual heat death of the universe and Timelike Infinity singularity of the future.[9] An omnibus edition of the first four Xeelee novels (Raft, Timelike Infinity, Flux, and Ring), entitled Xeelee: An Omnibus, was released in January 2010.[10] In August 2016, the entire series of all novels and stories (up to that date) was released as one volume in e-book format entitled Xeelee Sequence: The Complete Series.[11] Baxter's Destiny's Children series is part of the Xeelee Sequence.

Contents

Baxter first conceived of the Xeelee while hobby writing a short story in the summer of 1986 (that would later be published in Interzone as "The Xeelee Flower" in 1987). He incorporated powerful off-stage aliens to explain the story’s titular artifact, and in pondering the backstory began to flesh out the basics of what would later become the main players and setting of the Sequence: a universe full of intelligent species that live in the shadow of the incomprehensible and god-like Xeelee.[12]

In the Xeelee sequence life and intelligence are ubiquitous throughout the universe. The universe has a fundamental tendency toward complexity and autocatalystic systems are easily created. This is not limited to particle matter, but also applies more exotic structures such as dark matter, quarks, and flaws in space-time itself. Thus, life emerged when the universe was extremely young, to within Planck Time of the Big Bang, and there were intelligent civilizations that witnessed, and indeed consciously influenced, major events in the early universe such as inflationary expansion, symmetry breaking, and various phase changes. At the close of each of these epochs, as the universe took on a fundamentally different characteristic, there was a corresponding mass extinction, but small enclaves of the intelligences would survive in one manner or another, witnessing the universe becoming progressively colder and slower-reacting.

The novel Exultant describes the progenitor of the Xeelee, referred to as the proto-Xeelee, as one of the few survivors of the last phase change (when the universe had cooled enough for atoms to form). The proto-Xeelee established symbiotic relationships with other survivors as well as autocatalystic reactions in the new baryonic matter, thus emerging what would later be called the Xeelee. This potentially dates the Xeelee to when the universe was only a few hundred thousand years old, but with direct knowledge and observations that reaches back to within minutes of the big bang.

The Xeelee use primordial black holes as a habitat, construction tool, and computing devices because, at the time they emerged, the event horizon was one of the few thing that survived from earler epochs that they understood. More so, heavier atoms were exceedingly rare because stars had not yet had the time to create them through nuclear fusion. As the primordial black holes evaporated over time the Xeelee eventually migrated to using the supermassive black holes around which Galaxies were forming. Although the galactic black holes were trillions of times more massive than their ancestral homes, the Xeelee proved to be quite adept at using the complex physics around the event horizon for their own purposes. Over the billions of years since their migration, the Xeelee have become ubiquitous part of the complex ecology of the super massive black holes along with an unnamed number of other species that are also veterans of the same very early eras of the universe.

There is no explicit physical description of the Xeelee throughout the series. In the novel Exultant it is alluded that over their exceedingly long history, the Xeelee have combined themselves with their technology and, as such, may not have a distinct individual presence that Humans would be familiar with, but in Vacuum Diagrams, it is implied that they have small fingers, as the buttons and controls of their technology are small.

The Photino Birds are a species of dark matter with that prefer a habitat in the gravity well of stars. In general, the photino birds are probably unaware of almost all other forms of life due to the fact that dark matter interacts extremely weakly with baryonic matter (normal matter that is composed of atoms). The Photino Birds recognize a risk to their habitats from supernovas and other consequences of normal stellar evolution and have set themselves to ceasing nuclear fusion in the cores of stars, thereby prematurely ageing them into white dwarves. The resulting dwarves will have the same gravity well but be stable for billions of times longer than their progenitor. Because white dwarves are extremely cold and dim compared to typical stars, they would not support life on nearby planets. The long project of the Photino Birds will ultimately make the universe hostile to baryonic life, and they have spread across the universe. Further, the activities of the Photino Birds will effectively halt formation of new black holes due to a lack of Type II (core-collapse) supernova. This effectively puts the Xeelee and the Photino Birds at irreconcilable odds because each species is fundamentally seeking to threaten the ideal habitat of the other.

The Xeelee became aware of the Photino Birds approximately 13.5 billion years ago. Seemingly as a unified species, the Xeelee have concerned themselves with defeating the Photino Birds for almost the entire history of the universe. Despite their state of high technological advancement, the Xeelee were ultimately unsuccessful.

While recovering from a brutal occupation by the Qax that enforced stagnation and flirted with extinction, Humanity adopted an extremely xenophobic imperative that aimed to ensure the future of the species (known as the Druz Doctrines after its founder, Hama Druz). Eventually Humanity again began to expand into the galaxy, wiping out any other species encountered during their advance. Eventually Humans became the second most advanced and widespread race in the galaxy, second only to the Xeelee. Humans had no knowledge of the Photino-Xeelee war or even what was at stake, making the Xeelee seem inherently sinister. Building the Ring had the appearance of being destructive on a galactic scale because its larger purpose was not understood until the very far future, and many smaller Xeelee projects, such as changing the orbit of Callisto to preserve the bacteria there, lacked an obvious explanation. Though there were several orders of magnitude fewer Xeelee than humans known to be in the Galaxy, their high degree of technological advancement made them a formidable enemy.

Through a bitter war of attrition, Humans eventually contained the Xeelee within the Galactic Core where the battle lines stagnated for at least three thousand years. Both humans and the Xeelee gained strategic intelligence through time travel by constructing Closed Timelike Curves, further contributing to the stagnation. Human casualties were estimated to exceed more than 10 billion per year and more than 30 trillion over the course of the assault. The imperative commitment to total war also stagnated human physical and intellectual evolution. The Druz Doctrines did not encourage exploration or research and there was little new development. Further,very little effort was directed toward projects that did not have an immediate and conspicuous benefit to maintaining the massive logistics train necessary to continue the assault.

Eventually the Humans developed a means of hiding their actions from the Xeelee by making movable pocket universes that had no causal relationship to the universe as a whole, thereby eliminating any advantage of moving information into the past with time travel. Humans further developed a time-travel based computer that allowed humans arbitrary computer power to outthink the Xeelee by analysing tactical problems (giving the pilot tactical information to defeat Xeelee manouevres) and a dual black-hole cannon that would fire two singularities to merge at a known distance away, releasing enormous gravity waves in the process. Knowing that the ecology around the event horizon and the creatures that live within it would be damaged or destroyed if humans continued to batter the event horizon of the supermassive black hole, the Xeelee withdrew from the Milky Way within minutes after the humans scored the first successful hit.

No longer having the Xeelee as a unifying threat, humanity fragmented into multiple governments over the next several hundred years. The doctrinal stagnation collapsed and Humanity advanced technologically over the next 100,000 years. Humanity then unified again to begin directly attacking Xeelee concentration in the greater super-cluster of galaxies across the local group. Humans proved only a slight annoyance or distraction to the Xeelee.

As far as is known and aside from the Photino Birds, the Xeelee were seldom warlike and usually treated the junior species with disinterest or abandon. There are very few reported instances of them initiating interaction with another species. The Xeelee, however, demonstrated several instances of compassion and even charity toward other species, particularly when it involved escaping the Photino Bird's destruction of the universe. In the conclusion to novel Ring, set 5 million years in the future, they did not interfere with the humans escaping to another universe despite hundreds of thousands of years of humanity's war on the Xeelee (some interpretations are that they even provided an armed escort for the humans in the final leg of the voyage). Even though they were thought to have been driven to extinction by the humans, the Xeelee constructed a universe for the Silver Ghosts that was appealing to their mathematical nature. Before their own retreat from the universe, the Xeelee left ships as 'life boats' at various locations so that other species could use them to escape the Photino Birds through the Ring.

The Anti-Xeelee is a universe spanning quantum-wave consciousness constructed by the Xeelee to manage their projects. It was first briefly described in the conclusion of Timelike Infinity when it was encountered by Michael Poole after having been pushed several million years into the future. It was later developed somewhat more in the short story Vacuum Diagrams. Its name is derived from the particle/anti-particle naming convention to convey its paired and complimentary existence with the Xeelee; it does not describe an opposition to the Xeelee. It further implied that the Anti-Xeelee, in at least some regards, is travelling backwards in time like some anti-particles are thought to do.

The Xeelee were masters of time and space. They crafted their starships, known as Nightfighters, out of a strange form of matter that seems to violate the Pauli Exclusion Principle (which states that no two fermions can occupy the same quantum state). The material is incredibly sharp, hard, and transparent; due to its unique spacetime construction, it has strange gravitational effects.

Starbreakers are beams of coherent gravitational waves that the Xeelee use as both weapons and tools. They are consistently described as thin beams of red light that emit from Nightfighters and other Xeelee vessels encountered around Bolder's Ring. In the short story Blue Shift, the source of a Starbreaker is described as a pistol-like device designed for a hands of the size of a human child's. There is intricate wiring around the handle and an adjustment knob on the back. It will emit synchrotron radiation on the lowest setting, suggesting that it is powered by some form of particle acceleration.

Nearing the end of their work on the Ring, roughly 4 million years ago, the Xeelee built several hundred enclosures to return to the beginning of their history 20 billion years ago. These enclosures, called Sugar Lumps, appear as enormous cubes varying in size from a few thousand to one kilometer on a side (capable of enclosing moons or planets). More than a simple Closed Timelike Curve, the Sugar Lumps are described as having a negative vector in time (i.e. traveling backwards in time via a time symmetry) and function as inverse time capsules. When the Sugar Lumps arrived at their destination in the distant past, the Xeelee would emerge as a fully developed breed with their knowledge of future events and be able to immediately initiate their grandest projects against the Photino Birds.

Use of the Sugar Lumps allows the user to consciously manage their own evolution, and the evolution of other species as much as possible. It is therefore likely that the Xeelee were unable to prevent the evolution of the Photino Birds.

The Xeelee's greatest achievement — possibly the greatest structure of baryonic matter ever made — is the Ring, a gigantic loop of cosmic string millions of light-years across and spinning at close to light speed. Because of its intense gravitation and spinning, it creates a tear in the universe at its center, allowing ships to pass through and enter other universes.

The novels in chronological order (as opposed to publication order) are given below. It should be noted that some of the novels contain elements occurring at different points in the timeline. The story anthologies (Vacuum Diagrams, Resplendent, and Xeelee: Endurance) contain stories taking place across the entire chronology.

In 2009, Baxter posted a detailed chronology of the Xeelee Sequence explaining the proper chronological reading order of all the novels, novellas, and short stories up to that year. The timeline was updated in September 2015.[21]

When asked directly for a suggested reading order, the author wrote: "I hope that all the books and indeed the stories can be read stand-alone. I’m not a great fan of books that end with cliff-hangers. So you could go in anywhere. One way would be to start with ‘Vacuum Diagrams’, a collection that sets out the overall story of the universe. Then ‘Timelike Infinity’ and ‘Ring’ which tell the story of Michael Poole, then ‘Raft’ and ‘Flux’ which are really incidents against the wider background, and finally ‘Destiny’s Children.’"[22]

Baxter doesn’t shrink from tackling the dismayingly inhuman implications of vast abysses of past or future time, but the universality of life introduces perspective, motion and plot into every part of his Stapledonian cosmological framework.

It is great, heady, mind-bending stuff, meticulously mapped onto cutting edge speculations about the birth pangs of the universe and the ultimate fate of all known time and space, constantly enlivened and driven forward by the narratives that its vast range of life generates.

[It represents an] accomplished and imaginative exploration, expansion and reworking of SF’s core themes. His characters contest for living space with a panoply of bizarre aliens in a galaxy crammed with ancient wonders and secret histories; his stories reinvent the baroque excesses of space opera and brace them with imaginative exploration of ideas from stellar zoology, cosmology, quantum theory, exotic mathematics, and much else. Narratives froth with moments of shock and awe, and those sudden reversals of scale that induce the metaphysical dizziness sometimes called sense of wonder. Sentences stride confidently across centuries; paragraphs encompass millennia. Individual voices carry the story forwards, but the story is always bigger than the individuals that are caught up in it.[23]